1. Field of the Invention
The present invention relates to a site diversity operating method, and for example, to a site diversity operating method in a VDL Mode 3 system which is to be introduced into air traffic control services.
2. Related Art
For example, air traffic control services are currently carried out based on remote communications using analog voice communications, that is, so-called radio communications. Frequencies allocated to air traffic control internationally, within the VHF (Very High Frequency) band, are 118 to 137 MHz, which are divided into a plurality of channels. However, as demands for aviation communications increase in recent years, a trouble may be caused in the air traffic control due to the number of channels being in short.
In view of the above, there has been proposed a VHF digital link (VDL) system which substitutes for the analog voice communications. The VHF digital link is a next generation communication system, and standardizing operation for the system has been carried out in the ICAO (International Civil Aviation Organization).
As a VHF digital link, VDL Mode 3 has been standardized. VDL Mode 3 is a system substituting for the current radio communications, in which voice and data are digitally exchanged bi-directionally in addition to analog voice communications.
In performing site diversity operation in the VDL Mode 3, it is required to utilize the characteristics of the VDL Mode 3 as much as possible in such a situation that an operating mode similar to the analog ground-to-air radio system currently introduced is desired. For example, in order to conform to the operation of analog ground-to-air radio system, voice transmission from an air traffic controller is performed only from a ground site selected by the air traffic controller. For voice reception, there is required a system for superposing voice received from all ground sites and transmitting voice receivable as much as possible to air traffic controllers. In order to utilize the characteristics of the VDL Mode 3, it is required to surely provide services such as data link communications, voice source identification, an anti-blocking function and an air traffic controller priority voice without a waste, which become possible in addition to the conventional voice service.
Non-patent Document: The Fundamental Experiments of VHF Digital Link Mode 3 System (Electronic Navigation Research Institute Papers, ISSN 1341-9102, No. 108, issued on Jan. 16, 2004)
In providing these services, a control of beacon signals called management burst outputted from ground sites is deeply involved. In the case of site diversity operation, however, a plurality of ground sites for transmission and reception are provided and service provision is maintained while sharing the same frequency, so the management burst itself must be transmitted in a time dividing manner. Therefore, temporal delay is caused due to the positional relationship between aircrafts and ground sites.
Originally, a VDL Mode 3 system is not designed to take into account a delay problem of management burst caused in the case of controlling a plurality of ground sites like site diversity operation. It is only recently that an algorithm of management burst by site diversity and draft revisions of manuals considering delay problem of management burst are under preparation partially.
However, for practical operation, it is required to consider problems in management burst control and problems when transmitters and receivers are separated into plural devices, and to take measures against the problems.